Ribonucleotide reductases (RR) are evolutionarily conserved enzymes that catalyze the reduction of ribonucleotide diphosphates (rNDPs) to deoxyribonucleotide diphosphates (dNDPs), which is critical in the production and maintenance of dNTP pools. Orthopoxviruses encode genes for both large (˜90 kDa) and small (˜40 kDa) RR subunits, and homodimers of large and small subunits interact to form a functional RR complex.
Studies of vaccinia RR proteins found that insertional inactivation of I4L in strain WR did not cause observable defects in replication in culture and only mildly-attenuated these viruses in mouse models with an approximate 10-fold increase in lethal dose 50 values for this ΔI4L strain compared to wild-type virus (6). Lee et al. (23) reported a deletion mutant of 180 bp in the NYCBH and Wyeth strains of vaccinia although the specific sites of this deletion within the F4L (or R2) gene are not reported. These authors report that when the growth of this mutant was assessed in BSC-40 cells at a multiplicity of infection (MOI) of 10, this deletion mutant replicated with similar kinetics and yields to the parental (wild-type) strain although the actual quantitative data are not reported by the authors (23). These authors also report that this deletion mutant replicated to similar titers in mouse skin (23).
Vaccinia and other poxviruses have been used clinically. For example vaccinia virus has been used as a vaccine for smallpox. In addition, vaccinia virus has been investigated as an oncolytic virus for cancer therapy.